1. Field of the Invention
The present invention relates to a packet switching apparatus and, more particularly, to a gateway for connecting an access network to which user or subscriber terminals are connected and the Internet or an Internet service provider.
2. Description of the Related Art
There are various types of communication lines for accessing the Internet from a terminal such as a personal computer. A dial-up access to the Internet is shifting from a method using an analog telephone line to the ISDN (Integrated Services Digital Network) method enabling higher-speed communication. Recently, access methods, such as a method of using an ADSL (Asymmetric Digital Subscriber Line) and an optical access method using an FTTH (Fiber To The Home) which have a communication speed of 1 Mbit/s or higher are also coming into wide use. Other than an access from a fixed terminal installed at home, office, or the like, the Internet can be accessed also from a mobile terminal such as a portable telephone.
A packet switching apparatus (hereinbelow, called an access node) for relaying a plurality of different kinds of communication lines of different access methods to the Internet has to have not only common processing functions common to the various access methods but also individual processing functions peculiar to each of the access methods.
Examples of the processes common to the various access methods are a PPP (Point to Point Protocol) process for authenticating a subscriber, determining a communication path, and the like, an IP (Internet Protocol) layer process for determining the destination of a packet, and a switching process for switching a packet to a proper output interface determined by the destination. Examples of the processes peculiar to the access methods are a modem (modulation and demodulation) process in an analog line dial-up, and a process of a DSLAM (Digital Subscriber Line Access Multiplexer) in the ADSL.
In addition to a change in the access method, communication service in the Internet is also becoming various. Recently, a VoIP (Voice over Internet Protocol) for performing voice communication on the Internet is also coming into use. In the VoIP communication, it is necessary to convert a voice signal to an IP packet in an access node (gateway) for connecting a leased line network or PSTN (Public Switched Telephone Network) and the Internet. As a voice coding method, other than the PCM (ITU-T G.711) for transferring a voice signal at 64 kbits/s, for example, compression methods using different bandwidth such as 5.3 kbits/s, 6.4 kbits/s, 16 kbits/s, and 24 kbits/s have been standardized. In the VoIP communication, a process function for an echo canceller is required to each access node in order to prevent deterioration in voice quality.
As described above, an access node of the Internet has to conform to various access methods determined by communication service or a subscriber connection line. In this case, the hardware scale for realizing the individual process varies according to the access method and the type of communication service. Consequently, in the case of making a type of access node conform to a plurality of kinds of access methods, for example, in an access node supporting both a first access method requiring a small hardware scale for its individual process and a second access method requiring a large hardware scale for an individual process, the whole apparatus size depends on the hardware scale of the second access method.
FIG. 12 shows an access node supporting a first access method of a dial-up access using an analog line and a second access method of a dial-up access using the ISDN circuit.
This access node includes a plurality of line interface boards 1-1 to 1-n of the first access method for accommodating analog circuits L11 to Ljn, a line interface board 1-m of the second access method for accommodating an ISDN circuit Lm, a line interface board 2 for connecting to an ISP (Internet Service Provider) network, and a switch 3 for switching packets among the line interface boards.
In the case of making a dial-up access through an analog circuit, the access node has to perform modem termination processes. As modem standards for an analog line, for example, there are various protocols according to communication speeds such as V.90, K56 Flex, V.34+, and the like. Modem processes adapted to these protocols are usually realized by software. In order to accept dial-up accesses of a plurality of channels via analog circuits, a plurality of processors have to be provided in correspondence with the communication lines (channels), and a number of large size or small size of line interface boards adapted to the communication lines have to be inevitably required.
In FIG. 12, a physical layer termination unit 4 for connecting to the analog circuits L11 to L1n and a plurality of processors P11 to P1n corresponding to the analog circuits are mounted on the single line interface board 1-1. With the configuration, for example, by using a 60-by-60 cm board, modem processes of about 150 channels can be performed. For example, in the case of performing the modem processes of about 1000 channels by a single access node, it needs a hardware scale of about seven boards each having the above size.
On the other hand, a dial-up access via the ISDN circuit does not have to conform to a plurality of protocols unlike the modem processes in the analog circuit and does not need a converting process between an analog signal and a digital signal (A/D conversion and D/A conversion) as in the modem process. The line interface board 1-m for the ISDN circuit consequently has a relatively simple structure including the physical layer termination unit 4 and an ISDN termination unit 5. For example, when one channel is calculated as 64 kbits/s, a circuit process of about 4000 channels can be performed with a single board.